This is how I entertain myself in seeing the personality of his sister.
That she is pretty much as fond of Shouya as his mother is, "Geez, he's even cute when he's mad." (chapter 1)
He even emulates his sister's principle of War Against Boredom, so they must've been rather close, though not shown explicitly in the story.
The way I see it, she pretty much dumps Genki right after he messes with her brother and Pedro's first words to Shouya was that he'll cover his ass, rather odd sounding unless you assume that it's his sister that put that edict to Pedro (and seeing how his niece has thick Brazilian blood, it's safe to say Pedro did a good job of keeping to the edict since she has more than 30+ relationships before him).
This type of personality seemed to cement itself in me when she trusts her brother to take care of her daughter and the cat pouch scene where everyone seemed to notice the feelings Shouya himself seems oblivious to (everyone grins, meaning his sister should've been doing the same too).
Nobody mentioning the sister might've just been the author's cue that he's isolating himself from everyone, including his family. This is a story from his POV after all.